This invention relates to a process for cracking a hydrocarbon feedstock. In another aspect, the invention relates to a process for passivating contaminants on a cracking catalyst. In still another aspect, the invention relates to a cracking catalyst.
Contaminants, for example, nickel, vanadium and iron are found in significant concentrations in heavy oil fractions and lower quality crude oil. These contaminants have a poisoning effect on the expensive cracking catalysts employed to convert crude oil into gasoline and other valuable petroleum products, frequently making exploitation of these oils economically unattractive. Unfortunately, because of limited supplies of oils containing low levels of contaminants, it is sometimes necessary to employ the metals-contaminated oils in catalytic cracking processes.
The contaminants found in feedstocks to cracking processes become deposited on the cracking catalyst. The deposition on the catalyst of, for example, nickel, vanadium and iron, causes a decrease in the activity of the cracking catalyst to convert the hydrocarbon feedstock into cracked products, including gasoline. The selectivity of the cracking catalyst for cracking the feedstock into gasoline, as manifested by the portion of cracked products comprising gasoline, is also decreased. The production of undesirable products, for example, hydrogen and methane, which must be compressed, necessitating additional equipment; and coke, which is deposited on the catalyst and must be burned off, requiring additional equipment and "off-time" during which the catalyst is not employed for cracking, is significantly increased.
Because of these problems, the industry usually replaces cracking catalysts contaminated by more than about 3,000 parts per million (ppm) of vanadium equivalents of vanadium and nickel, defined as the sum of the parts by weight of vanadium and four times the parts by weight of nickel in one million parts by weight of contaminated cracking catalyst. This level of contamination is rapidly reached when cracking heavily contaminated feedstocks. There is thus a need for a cracking process suitable for use with contaminated feedstocks and contaminated cracking catalysts. There is also a need for a cracking catalyst which is only minimally adversely affected by deposits thereon of contaminants selected from nickel, vanadium and iron. There is also a need for a process of treating a contaminated cracking catalyst to increase its activity for conversion and selectivity for producing gasoline and to decrease its selectivity for producing undesirable products, for example, hydrogen and coke.